The concept of turning your health into ammo is really cool, but the game is way too easy, and there isn't really enough variety in enemies. You can just stand in one place, shooting at everything that approaches, and only move to pick up health - which is abundant. It's almost impossible to die, unless you decide to.
I would suggest adding enemies that can shoot at you, or maybe ones that can't be killed head-on, forcing you to flank them. SOMETHING that would force the player to more actively maneuver around the map